The proposed experiments are designed to study the aggressive behavior of pregnant and of lactating mice. The studies are divided into three major sections. Those of the 1st are concerned with postpartum aggression. Animals that do and do not exhibit such behavior will be compared with respect to the exhibition of other aspects of maternal behavior and lactation performance. Also, an attempt is made to determine why aggression is not displayed within the first 48 hours following parturition. Experiments of the 2nd section, dealing with agonistic behavior of pregnant mice, compare prepartum with postpartum aggression and attempt to understand the mechanism controlling aggressive behavior of pregnant animals. Finally, experiments of the 3rd section test whether a lack of maternal aggression is one of the mechanisms which limit population size and, in addition, whether crowding exerts permanent affects upon maternal aggression.